1. Field of the Invention
The present invention relates generally to a system including method and apparatus for measuring the torque transmitted by a shaft and, in particular, a system which automatically compensates for variations in wall thickness exhibited by different drive shafts having the same nominal dimensions.
2. Description of the Prior Art
The present invention is particularly suited for use with monopole variable reluctance torque measuring apparatus of the type disclosed in U.S. Pat. Nos. 3,548,649 and 4,488,443, although its use need not be so restricted.
As disclosed in these earlier patents, the prior art has included various systems for measuring transmitted shaft torque by measuring the "twist" or torsional deflection of a length of the shaft while it is under a torsional load. Typically, a pair of toothed wheels are physically attached to spaced apart respective locations along the shaft and the relative displacement between the teeth of these two wheels is then detected by a suitable electrical signal transducer. For example, photo-electric or variable reluctance sensors may be utilized to detect the relative positions of teeth in the two toothed wheels.
In one monopole sensor embodiment, the toothed wheels are physically located substantially adjacent to one another although the relative rotational position of at least one of the wheels is determined by a tubular sleeve structure actually affixed to the shaft at some more significant distance away from the other wheel. In this way, sufficient shaft length is included so as to produce a desired magnitude of relative rotational movement between the toothed wheels when the shaft is torsionally loaded to a predetermined magnitude. In one such monopole sensor embodiment, the teeth are, at least in part, axially extended with the teeth on one wheel being interleaved between those on the other wheel and with a single pole variable reluctance sensor disposed to monitor the overlapped portions of such teeth as they rotate therepast. This arrangement is depicted in the exemplary embodiments of the above-referenced related patents.
The monopole sensor produces a voltage pulse as each target tooth crosses its path. Actually, a string of voltage pulses is produced over a period of time as the shafts rotate teeth past the sensor. The phase relation of these pulses is caused to shift as the drive shaft twists under load and moves its target teeth. Calculation of torque is made with a digital computer software program which correlates the measured twist or phase shift of the drive shaft with the magnitude of the torque being transmitted through the shaft. This correlation is based on the geometry of the shaft and its elastic modulus at a given temperature. Torque measurement accuracy is dependent upon both characteristics. However, temperature is measured and modulus is consistent from one shaft to another. Therefore, most variations in accuracy of torque measuring apparatus previously in use can be attributed to variations in shaft geometry (that is, wall thickness) from unit to unit in large production lots.
Conventionally, calibration of the torquemeter and shaft system has been made by a "zero" adjustment to the target teeth in a calibration rig, and a "slope" correction which groups shafts into three-to-five "classes" based on the amount of twist imparted to the shaft under a standardized load. In other words, a shaft which has been fabricated with walls machined to the thin side of a manufacturing tolerance will twist more for a given torque load than a shaft produced with walls to the thick side of that manufacturing tolerance. Therefore, the correlation characteristic used in the computer software will be different for the thin-walled class of shafts than for the thick-walled class of shafts. Current practice is to classify production shafts during calibration, and provide an adjustment to the calculation during assembly by means of a switch or resistor external to the digital computer. The drawback of this system is that the shaft and computer become a matched set of parts, and when one is removed and replaced, an adjustment is mandated.
As will be appreciated, the need to manually program the signal processing circuitry to compensate for a particular shaft in a particular installation not only imposes added administrative overhead to successful system operation, it also increases the chance that an erroneous signal compensation will in fact be effected if the manually entered compensation data is not properly coordinated with the actual shaft in a given installation. This includes situations where a different set of electronics may be associated with a given shaft during field installation and/or maintenance procedures.
A recent attempt to avoid the aforesaid difficulty of matching the shaft and computer is presented in U.S. Pat. No. 4,602,515 according to which torque measurement apparatus of this general type contains embedded data structures which can be used to provide automatic compensation for torque measurement errors induced by variations in gap, speed and shaft modulus among individual torque measuring systems. This modified apparatus may also conventionally provide temperature compensation.
According to this recent prior art development, differential tooth height, that is, the radial tooth dimension, is used to encode shaft modulus data while simultaneously providing a differential amplitude modulation component related to gap size. Since signal components related to speed are also inherently present, and since the differential pulse amplitude corresponds to the encoded shaft modulus data, the signal processing circuitry of the exemplary torque meter is also enabled to automatically compensate for errors induced by gap size between adjacent teeth, speed and shaft modulus for any given system.
Since the shaft modulus data unique to a given shaft is permanently and physically embedded within the toothed wheels at the time of manufacture, such automatic shaft modulus compensation will necessarily be correctly entered into any appropriate signal processing circuitry connected thereto in the field during installation and/or maintenance procedures. Of course, to achieve this desired result, it is necessary to use some form of "intelligent" signal processor such as a programmed microproccesor.
A significant drawback of this system, however, resides in the fact that each individual shaft must be modified in a specialized manner which differentiates it from each other shaft.